Note: Descriptions are shown in the official language in which they were submitted.
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POLY(ETHYLENE CHLOROTRIFLUOROETHYLENE) MEMBRANES
TECHNICAL FIELD
The invention relates to Halar , (ethylene chlorotrifluoroethylene copolymer,
or
poly (ethylene chlorotrifluoroethylene)) and related membranes for use in
ultrafiltration and
microfiltration and in particular to membranes in the form of hollow fibres,
and to methods
of preparing said membranes. Halar is a registered trade mark.
BACKGROUND ART
The following discussion is not to be construed as an admission with regard to
the
common general knowledge in Australia.
Synthetic polymeric membranes are well known in the field of ultrafiltration
and
microfiltration for a variety of applications including desalination, gas
separation, filtration
and dialysis. The properties of the membranes vary depending on the
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morphology of the membrane i.e. properties such as symmetry, pore shape, pore
size
and the chemical nature of the polymeric material used to form the membrane.
Different membranes can be used for specific separation processes, including
microfiltration, ultrafiltration and reverse osmosis. Microfiltration and
ultrafiltration are
pressure driven processes and are distinguished by the size of the particle or
molecule
that the membrane is capable of retaining or passing. Microfiltration can
remove very
fine colloidal particles in the micrometer and submicrometer range. As a
general rule,
microfiltration can filter particles down to 0.05 pm, whereas ultrafiltration
can retain
particles as small as 0.01pm and smaller. Reverse Osmosis operates on an even
smaller
scale.
Microporous phase inversion membranes are particularly well suited to the
application of removal of viruses and bacteria.
A large surface area is needed when a large filtrate flow is required. A
commonly used technique to minimize the size of the apparatus used is to form
a
membrane in the shape of a hollow porous fibre. A large number of these hollow
fibres
(up to several thousand) are bundled together and housed in modules. The
fibres act in
parallel to filter a solution for purification, generally water, which flows
in contact with
the outer surface of all the fibres in the module. By applying pressure, the
water is
forced into the central channel, or lumen, of each of the fibres while the
microcontaminants remain trapped outside the fibres. The filtered water
collects inside
the fibres and is drawn off through the ends.
The fibre module configuration is a highly desirable one as it enables the
modules to achieve a very high surface area per unit volume.
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In addition to the arrangement of fibres in a module, it is also necessary for
the
polymeric fibres themselves to possess the appropriate microstructure to allow
microfiltration to occur.
Desirably, the microstructure of ultrafiltration and microfiltration membranes
is
asymmetric, that is, the pore size gradient across the membrane is not
homogeneous, but
rather varies in relation to the cross-sectional distance within the membrane.
Hollow
fibre membranes are preferably asymmetric membranes possessing tightly bunched
small pores on one or both outer surfaces and larger more open pores towards
the inside
edge of the membrane wall.
This microstructure has been found to be advantageous as it provides a good
balance between mechanical strength and filtration efficiency.
As well as the microstructure, the chemical properties of the membrane are
also
important. The hydrophilic or hydrophobic nature of a membrane is one such
important
property.
Hydrophobic surfaces are defined as "water hating" and hydrophilic surfaces as
"water loving". Many of the polymers used to cast porous membranes are
hydrophobic
polymers. Water can be forced through a hydrophobic membrane by use of
sufficient
pressure, but the pressure needed is very high (150-300 psi), and a membrane
may be
damaged at such pressures and generally does not become wetted evenly.
Hydrophobic microporous membranes are typically characterised by their
excellent chemical resistance, biocompatibility, low swelling and good
separation
performance. Thus, when used in water filtration applications, hydrophobic
membranes need to be hydrophilised or "wet out" to allow water permeation.
Some
hydrophilic materials are not suitable for microfiltration and ultrafiltration
membranes
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that require mechanical strength and thermal stability since water molecules
can play
the role of plasticizers.
Currently, poly(tetrafluoroethylene) (PTFE), polyethylene (PE), polypropylene
(PP) and poly(vinylidene fluoride) (PVDF) are the most popular and available
hydrophobic membrane materials. PVDF exhibits a number of desirable
characteristics
for membrane applications, including thermal resistance, reasonable chemical
resistance
(to a range of corrosive chemicals, including sodium hypochlorite), and
weather (UV)
resistance.
While PVDF has to date proven to be the most desirable material from a range
of materials suitable for microporous membranes, the search continues for
membrane
materials which will provide better chemical stability and performance while
retaining
the desired physical properties required to allow the membranes to be formed
and
worked in an appropriate manner.
In particular, a membrane is required which has a superior resistance
(compared
to PVDF) to more 4gressive chemical species, in particular, oxidising agents
and to
conditions of high pH i.e. resistance to caustic solutions. In particular with
water
filtration membranes, chlorine resistance is highly desirable. Chlorine is
used to kill
bacteria and is invariably present in town water supplies. Even at low
concentrations, a
high throughput of chlorinated water can expose membranes to large amounts of
chlorine over the working life of a membrane can lead to yellowing or
brittleness which
are signs of degradation of the membrane.
Microporous synthetic membranes are particularly suitable for use in hollow
fibres and are produced by phase inversion. In this process, at least one
polymer is
= dissolved in an appropriate solvent and a suitable viscosity of the
solution is achieved.
The polymer solution can be cast as a film or hollow fibre, and then immersed
in
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precipitation bath such as water. This causes separation of the homogenous
polymer
solution into a solid polymer and liquid solvent phase. The precipitated
polymer forms a
porous structure containing a network of uniform pores. Production parameters
that affect
the membrane structure and properties include the polymer concentration, the
precipitation
media and temperature and the amount of solvent and non-solvent in the polymer
solution.
These factors can be varied to produce microporous membranes with a large
range of pore
sizes (from less than 0.1 to 2011m), and possess a variety of chemical,
thermal and
mechanical properties.
Hollow fibre ultrafiltration and microfiltration membranes are generally
produced by
either diffusion induced phase separation (the DIPS process) or by thermally
induced phase
separation (the TIPS process).
Determining the appropriate conditions for carrying out the TIPS process is
not
simply a matter of substituting one polymer for another. In this regard,
casting a polymeric
hollow fibre membrane via the TIPS process is very different to casting or
extruding a bulk
item from the same material. The TIPS procedure is highly sensitive, each
polymer
requiring careful selection of a co-solvent, a non-solvent, a lumen forming
solvent or non-
solvent, a coating solvent or non-solvent and a quench, as well as the
appropriate production
parameters, in order to produce porous articles with the desired chemically
induced
microstructure in addition to the overall extruded high fibre structure.
The TIPS process is described in more detail in PCT/AU1991/000198
(WO/1991/017204) AU 653528.
The quickest procedure for forming a microporous system is thermal
precipitation of
a two component mixture, in which the solution is formed by dissolving a
thermoplastic
polymer in a solvent which will dissolve the polymer at an elevated
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temperature but will not do so at lower temperatures. Such a solvent is often
called a
latent solvent for the polymer. The solution is cooled and, at a specific
temperature
which depends upon the rate of cooling, phase separation occurs and the
polymer rich
phase separates from the solvent.
All practical thermal precipitation methods follow this general process which
is
reviewed by Smolders et al in Kolloid Z.u.Z Polymer, 43, 14-20 (1971). The
article
distinguishes between spinodal and binodal decomposition of a polymer
solution.
The equilibrium condition for liquid-liquid phase separation is defined by the
binodal curve for the polymer/solvent system. For binodal decomposition to
occur, the
solution of a polymer in a solvent is cooled at an extremely slow rate until a
temperature
is reached below which phase separation occurs and the polymer rich phase
separates
from the solvent.
It is more usual for the phases not to be pure solvent and pure polymer since
there is still some solubility of the polymer in the solvent and solvent in
the polymer,
there is a polymer rich phase and a polymer poor phase. For the purposes of
this
discussion, the polymer rich phase will be referred to as the polymer phase
and the
polymer poor phase will be referred to as the solvent phase.
When the rate of cooling is comparatively fast, the temperature at which the
phase separation occurs is generally lower than in the binodal case and the
resulting
phase separation is called spinodal decomposition.
According to the process disclosed in U.S. Specification No. 4,247,498, the
relative polymer and solvent concentrations are such that phase separation
results in fine
droplets of solvent forming in a continuous polymer phase. These fine droplets
form the
cells of the membrane. As cooling continues, the polymer freezes around the
solvent
droplets.
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As the temperature is lowered, these solubilities decrease and more and more
solvent droplets appear in the polymer matrix. Syneresis of the solvent from
the
polymer results in shrinkage and cracking, thus forming interconnections or
pores
between the cells. Further cooling sets the polymer. Finally, the solvent is
removed from
the structure.
Known thermal precipitation methods of porous membrane formation depend on
the polymer rich phase separating from the solvent followed by cooling so that
the
solidified polymer can then be separated from the solvent. Whether the solvent
is liquid
or solid when it is removed from the polymer depends on the temperature at
which the
operation is conducted and the melting temperature of the solvent.
True solutions require that there be a solvent and a solute. The solvent
constitutes a continuous phase and the solute is uniformly distributed in the
solvent with
no solute-solute interaction. Such a situation is almost unknown with the
polymer
solutions. Long polymer chains tend to form temporary interactions or bonds
with other
polymer chains with which they come into contact. Polymer solutions are thus
rarely
true solutions but lie somewhere between true solutions and mixtures.
In many cases it is also difficult to state which is the solvent and which is
the
solute. In the art, it is accepted practice to call a mixture of polymer and
solvent a
solution if it is optically clear without obvious inclusions of either phase
in the other. By
optically clear, the skilled artisan will understand that polymer solutions
can have some
well known light scattering due to the existence of large polymer chains.
Phase
separation is then taken to be that point, known as the cloud point, where
there is an
optically detectable separation. It is also accepted practice to refer to the
polymer as the
solute and the material with which it is mixed to form the homogeneous
solution as the
solvent.
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In the present case the inventors have sought to find a way to prepare Halar
membranes without the use of highly toxic solvents, and in particular, to
prepare hollow
fibre Halar membranes. Halar, or poly (ethylene chlorotrifluoroethylene), is a
1:1
alternating copolymer of ethylene and chlorotrifluoroethylene, and having the
following
structure:
-(-CH2-CH2-CFC1-CF241-
While the embodiments of the invention are described herein with respect to
Halar, this term is used herein to encompass Halar equivalents, such as
-(-(CH2-CH2-).-CX2-CX2--)n-
wherein each X is independently selected from F or Cl, and where m is chosen
so as to
be between 0 and 1, so as to allow the ethylene portion of the polymer to
range from 0
to 50%. An example of a Halar equivalent is PCTFE.
It has been known for some time to produce flat sheet Halar membranes, and the
processes are disclosed in US 4702836, for example. The previous methods were
not
amenable to producing hollow fibres and moreover, utilised solvents which are
highly
toxic with high environmental impact, such as 1,3,5-trichlorobenzene, dibutyl
phthalate
and dioctyl phthalate.
The properties of Halar make it highly desirable in the field of
ultrafiltration and
microfiltration. In particular, Halar has extremely good properties in
relation to its
resistance both to chlorine and to caustic solutions, but also to ozone and
other strong
oxidising agents. While these desiderata have been established for some time,
it was
hitherto unknown how to fulfil the long felt need to make hollow fibre
membranes from
such a desirable compound. Further, a disadvantage in relation to the existing
prepararatory methods for Halar flat sheet membranes is that they require the
use of
highly toxic solvents or solvents that are of dubious safety at the very
least. For
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instance, the conventional state of the art is that the solvents needed are
aromatic
solvents such as dibutyl phthalate (DBP), dioctyl phthalate (DOP) and 1,3,5-
trichlorobenzene (TCB). Such difficult solvents are required due to the
chemical
stability of Halar and its resistance to most common solvents below 150 C.
It is an object of the present invention to overcome or ameliorate at least
one of
the disadvantages of the prior art, or to provide a useful alternative,
particularly in terms
of methods of production.
SUMMARY OF THE INVENTION
According to a first aspect, the invention provides a porous polymeric
ultrafiltration or microfiltration membrane including Halar and formed without
the use
of toxic solvents, or solvents of dubious or unproven safety.
The membranes may be preferably flat sheet, or, more preferably hollow fibres.
Preferably, the porous polymeric ultrafiltration or microfiltration membrane
is
formed by the TIPS (thermally induced phase separation) process and has an
asymmetric pore size distribution. Most preferably, the Halar ultrafiltration
or
microfiltration membrane has an asymmetric cross section, a large-pore face
and a
small-pore face.
Preferably, the porous polymeric Halar membrane has pore size is in the range
0.011.1m to 201.1m. Pore size can be determined by the so called bubble point
method.
According to a second aspect, the invention provides a porous polymeric
ultrafiltration or microfiltration membrane formed from Halar and prepared
from a
solution containing one or more compounds according to formula I or formula
II:
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COORi 00C R1
R4 ________________ COOR2 or R4 ________________ 00C R2
COOR3 -00CR3
I II
wherein R1, R2 and R3 are independently methyl, ethyl, propyl, butyl, pentyl,
hexyl or
other alkyl.
R4 is H, OH, COR5, OCOR5, methyl, ethyl, propyl, butyl, pentyl, hexyl or other
alkyl, methoxy, ethoxy, propoxy, butoxy, pentoxy, hexoxy or other alkoxy,
R5 methyl, ethyl, propyl, butyl, pentyl, hexyl or other alkyl.
Preferably, R1 R2 = R3 = ethyl and R4=H.
Preferably, the pore controlling agent is citric acid ethyl ester (CitroflexTm-
2) or
glycerol triacetate.
The above compounds may be used as polymer solvents, coating agents or both,
and may be used alone, in mixtures of the above compounds, or in conjunction
with
other appropriate agents.
The porous polymeric ultrafiltration or microfiltration membranes of the
present
invention may include one or more materials compatible with the Halar.
The porous polymeric membranes ultrafiltration or microfiltration of the
present
invention may be either hydrophobic or hydrophilic, and may include other
polymeric
materials compatible with Halar. Additional species adapted to modify the
chemical
behaviour of the membrane may also be added. In one highly preferred
alternative, the
porous polymeric membrane of the present invention further including modifying
agent
to modify the hydrophilicity / hydrophobicity balance of the membrane. This
can result
in a porous polymeric membrane which is hydrophilic or alternatively, a porous
polymeric membrane which is hydrophobic.
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According to a third aspect, the invention provides a porous polymeric
ultrafiltration
or microfiltration membrane formed from Halar and incorporating a leachable
agent.
In one preferred embodiment, the leachable agent is silica.
Preferably, the silica is present in an amount of from 10 to 50wt% of the
final
polymer, and more preferably around 30%. The silica may be hydrophobic silica
or
hydrophilic silica. Highly preferred are fumed silica's such as the
hydrophilic Aerosil 200
and the hydrophobic Aerosil R972. Aerosil is a registered trade mark.
Preferably, the porous polymeric ultrafiltration or microfiltration membranes
of the
present invention have one or more of the following properties: high
permeability (for
example, greater than 1000LMH/hr@100KPa), good macroscopic integrity, uniform
wall
thickness and high mechanical strength (for example, the breakforce extension
is greater
than 1.3N).
According to a fourth aspect, the present invention provides a method of
making a
porous polymeric material comprising the steps of:
(a) heating a mixture comprising Halar and a solvent system initially
comprising a
first component that is a latent solvent for Halar and optionally a second
component that is a
non-solvent for Halar wherein, at elevated temperature, Halar dissolves in the
solvent
system to provide an optically clear solution,
(b) rapidly cooling the solution so that non-equilibrium liquid-liquid phase
separation takes place to form a continuous polymer rich phase and a
continuous polymer
lean phase with the two phases being intermingled in the form of bicontinuous
matrix of
large interfacial area,
(c) continuing cooling until the polymer rich phase solidifies; and
(d) removing the polymer lean phase from the solid polymeric material.
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According to a fifth aspect, the invention provides a porous polymeric
ultrafiltration or microfiltration membrane formed from Halar and containing
silica and
wherein said polymeric porous Halar membrane has a coating of a coating agent
including of one or more compounds according to formula I or II:
COORi 00C R1
R4 _______________ COOR2 or R4 __________ 00CR2
COOR3 ___________________________________ 00CR3
I II
wherein R1 R2 and R3 are independently methyl, ethyl, propyl, butyl, pentyl,
hexyl or
other alkyl.
R4 is H, OH, COR5, OCOR5, methyl, ethyl, propyl, butyl, pentyl, hexyl or other
alkyl, methoxy, ethoxy, propoxy, butoxy, pentoxy, hexoxy or other alkoxy,
R5 methyl, ethyl, propyl, butyl, pentyl, hexyl or other alkyl.
Preferably, R1 = R2 R3 ethyl and R4=H.
Preferably, the pore controlling agent is an environmentally friendly solvent.
Preferably, the pore controlling agent is citric acid ethyl ester (CitroflexTm-
2) or
glycerol triacetate.
According to a sixth aspect, the invention provides a method of manufacturing
a
microfiltration or ultrafiltration membrane including the step of casting a
membrane
from a polymer composition including Halar
According to a seventh aspect, the invention provides a method of forming a
hollow fibre Halar membrane comprising:
forming a blend of Halar with a compatible solvent;
forming said blend into a shape to provide a hollow fibre;
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contacting an internal lumen surface of said blend with a lumen forming fluid;
inducing thermally induced phase separation in said blend to form a hollow
fibre
membrane; and
removing the solvent from the membrane.
Preferably, the Halar is present in the blend in an amount ranging from 14-
25%,
and most preferably around 16-23%. Preferably, the pore controlling agent is
an
environmentally friendly solvent, such as GTA or Citroflex 2. Preferably, the
lumen
forming fluid is digol. In highly preferred embodiments, the process is
conducted at
elevated temperatures, preferably above 200 C, and more preferably above 220
C.
According to an eighth aspect, the invention provides a method of forming a
hollow fibre Halar membrane comprising:
forming a blend of Halar with a compatible solvent;
forming said blend into a shape to provide a hollow fibre;
contacting an external surface of said blend with a coating fluid;
contacting an internal lumen surface of said blend with a lumen forming fluid;
inducing thermally induced phase separation in said blend to form a hollow
fibre
membrane; and
extracting the solvent from the membrane.
Preferably, the coating is selected from one or more of GTA, citroflex-2 and
digol.
According to an ninth aspect, the invention provides a method of forming a
hollow fibre Halar membrane comprising:
forming a blend of Halar with a compatible solvent;
suspending a pore forming agent in said blend
forming said blend into a shape to provide a hollow fibre;
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contacting an internal lumen surface of said blend with a lumen forming fluid;
inducing thermally induced phase separation in said blend to form a hollow
fibre
membrane; and
extracting the solvent from the membrane.
Preferably, the pore forming agent is a leachable pore forming agent, such as
silica.
According to a tenth aspect, the invention provides a method of forming a
hollow fibre Halar membrane comprising:
forming a blend of Halar with a compatible solvent;
suspending a pore forming agent in said blend
forming said blend into a shape to provide a hollow fibre;
contacting an external surface of said blend with a coating fluid;
contacting an internal lumen surface of said blend with a lumen forming fluid;
inducing thermally induced phase separation in said blend to form a hollow
fibre
membrane; and
extracting the solvent from the membrane.
Preferably the pore forming agent is a leachable pore forming agent, more
preferably silica. The method may further include the step of leaching said
leachable
pore forming agent from said membrane. Preferably, the pore forming agent is a
leachable silica, which is leached from the dope by caustic solution,
In certain preferred embodiments, the digol is used as a non-solvent and
independently water is used as a quench fluid.
According to an eleventh aspect, the invention provides a method of forming a
hollow fibre Halar membrane comprising:
forming a blend of Halar with a compatible solvent;
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suspending a leachable pore forming agent in said blend
forming said blend into a shape to provide a hollow fibre;
contacting an internal lumen surface of said blend with a lumen forming fluid;
inducing thermally induced phase separation in said blend to form a hollow
fibre
membrane;
extracting the solvent from the membrane; and
leaching said leachable pore forming agent from said membrane.
According to a twelfth aspect, the invention provides a method of forming a
hollow fibre Halar membrane comprising:
forming a blend of Halar with a compatible solvent;
suspending a leachable pore forming agent in said blend
forming said blend into a shape to provide a hollow fibre;
contacting an external surface of said blend with a coating fluid;
contacting an internal lumen surface of said blend with a lumen forming fluid;
inducing thermally induced phase separation in said blend to form a hollow
fibre
membrane;
extracting the solvent from the membrane; and
leaching said leachable pore forming agent from said membrane.
Preferably, the pore forming agent is a leachable pore forming agent, such as
silica,
which is leached from the dope by caustic solution, preferably Swt%
Preferably, digol is used as a non-solvent and independently water is used as
a
quench fluid.
According to a thirteenth aspect, the present invention provides the use of
Halar
for forming a hollow fibre ultrafiltration or microfiltration membrane.
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According to a fourteenth aspect, the present invention provides method of
forming a polymeric ultrafiltration or microfiltration membrane including the
steps of:
preparing a leachant resistant Halar membrane dope;
incorporating a leachable pore forming agent into the dope;
casting a membrane; and
leaching said leachable pore forming agent from said membrane with said
leachant.
Preferably, the leachable pore forming agent is an inorganic solid with an
average particle size less than 1 micron, and most preferably is leachable
silica. In
highly preferred embodiments, the silica is present in around 3-9%
Preferably, the leachant is a caustic solution.
The invention also provides a porous polymeric Halar microfiltration or
ultrafiltration
membrane when prepared by any of the preceding aspects.
According to a fifteenth aspect, the invention provides a microporous Halar
membrane prepared from an environmentally friendly solvent or mixture of
environmentally friendly solvents.
Preferably, the membrane is a flat sheet or hollow fibre membrane.
Preferably, the flat sheet membrane is prepared from an environmentally
friendly solvent or mixture of solvents containing one or more compounds
according to
the following formula:
COORi 00CR1
R4 _______________ COOR2 or R4 __________ 00CR2
COOR3 ___________________________________ 00CR3
wherein R1, R2 and R3 are independently methyl, ethyl, propyl, butyl, pentyl,
hexyl or
other alkyl.
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R4 is H, OH, COR5, OCOR5, methyl, ethyl, propyl, butyl, pentyl, hexyl or other
alkyl, methoxy, ethoxy, prop oxy, butoxy, pentoxy, hexoxy or other alkoxy,
R5 methyl, ethyl, propyl, butyl, pentyl, hexyl or other alkyl.
Preferably, R1= R2 = R3 = ethyl and R4=H.
Preferably, the pore controlling agent is citric acid ethyl ester (CitroflexTm-
2) or
glycerol triacetate.
The term "environmentally friendly" as used herein refers to materials having
a
lesser or reduced effect on human health and the environment when compared
with
competing products or services that serve the same purpose. In particular,
"environmentally friendly" refers to materials which have low toxicity to
plants and
animals, especially humans. Environmentally friendly also encompasses
biodegradable
materials.
Preferably, the environmentally friendly solvents used in the present
invention
are not recognised as hazardous to the health of humans or other organisms,
either when
subject exposure is acute (short term/high dose) or long term (typically at a
lower dose).
It is preferable, that the acute toxicity be low, ie it is preferable if the
solvents
have a high LDS . For example, the LD50 of glycerol triacetate in rodents is
around
3000mg/kg bodyweight, whereas in the case of 1,3,5-trichlorobenzene, the LD50
is as
low as 300-800mg/kg. Preferably in the present invention, the LD50 is above
1000mg/kg, and more preferably above 2000 mg/kg
However, as well as acute toxicity, it is also highly desirable that the
solvents do
not show long term, low level exposure effects, and are not carcinogenic,
mutagenic or
teratogenic. This will not so much be reflected by their LD5O's (although
these are a
factor), but reflects factors such as the ability of the solvent to
bioaccumulate as well as
its inherent toxic and mutagenic properties. Preferably, the solvents of the
present
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invention do not bioaccumulate. In this regard, the biodegradability of the
solvent is important,
and high biodegradability is preferred.
It is also necessary to consider other ecotoxicological effects such as the
toxicity to non-
humans/non-mammals, and factors such as whether the solvent is an ozone
depleting compound.
In terms of structural considerations, the type of structural features which
may be found
in suitable environmentally friendly solvents include the presence of
degradable groups, eg
hydrolysable groups, such as esters, (especially when these result in much
smaller molecules,
such as C4 or less); absence of halogens (such as chlorine); and the absence
of aromatic rings.
The preferred solvents of the present invention exhibit these three favourable
characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig la and lb are diagrams of alternative TIPS processes used to prepare HF
membranes.
Figs 2 and 3 are Scanning Electron Micrographs of the membranes of the present
invention.
Fig. 4 shows the results of IGG filtration using the membranes of the present
invention.
BEST METHOD OF PERFORMING THE INVENTION
The TIPS process is described in more detail in PCT/AU1991/000198
(WO/1991/017204) AU 653528. The current method used to prepare the membranes
of the
present invention is described herein in simplified form.
In one preferred form of the invention, poly (ethylene
chlorotrifluoroethylene) is formed
as a hollow fibre. The poly (ethylene chlorotrifluoroethylene) is dissolved in
a suitable solvent
and then passed through an annular co-extrusion head.
4221217
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There are two possible ways to conduct the methods of the present invention in
relation to hollow fibres. One is via a co extrusion head having three
concentric
passageways, as shown in cross section figure lb, the other is via a quadruple
co-
extrusion head having four concentric passageways is shown in cross section in
Figure
la. The principle is broadly the same in both cases, except for the way the
quench fluid
is contacted with the fibre.
In both cases, the axial passageway 1 may contain a lumen forming fluid 11.
The first outwardly concentric passageway 2 contains a homogenous mixture of
the
polymer and solvent system 12 to form the membrane, the next outwardly
concentric
passageway 3 has a coating fluid 13. In the case of the triple extrusion head,
the quench
is a bath either directly adjacent the extrusion head or slightly spaced below
it with an
intermediate air gap. In the quadruple extrusion head, the outermost
passageway 4
applies a quench fluid 14 to the fibre.
Under carefully thermally controlled conditions, the lumen forming fluid, the
membrane forming solution and the coating fluid are coating fluid are
contacted with a
quench fluid at a predetermined temperature (and flow rate, if the quench is
applied by
means of an outermost concentric passageway). The poly (ethylene
chlorotrifluoroethylene) solution comes into contact with the lumen forming
fluid on the
inside of the hollow fibre and with the coating fluid and/or quench bath
solution on the
outside of the hollow fibre.
The lumen and coating fluids contain one or more components of the solvent
system, alone or in combination with other solvents, in selected proportions
(the first
component may be absent). The composition of the coating and lumen fluids
predetermine the pore size and frequency of pores on the membrane surfaces.
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Each fluid is transported to the extrusion head by means of individual
metering
pumps. The three components are individually heated and are transported along
thermally insulated and heat traced pipes. The extrusion head has a number of
temperature zones. The lumen fluid, membrane forming solution (dope) and
coating
fluid are brought to substantially the same temperature in a closely monitored
temperature zone where the dope is shaped. As mentioned above, the exact
nature of
the quench depends on whether the quadruple or triple extrusion head is used.
In the
quadruple, the quench fluid is introduced via an outer concentric passageway.
The fibre
may travel down the quench tube at a significantly different linear speed from
the
quench fluid. The fibre may then pass into a further quantity of quenching
fluid if
desired.
In the triple extruder system, the fibre passes out of the die, which may be
optionally in the shape of a stem to assist in determining fibre structure.
The fibre may
pass through an optional air gap before passing into a quench bath. Most
fibres
disclosed herein were prepared by the triple extrusion head, as will be clear
by the
inclusion of an air gap distance in the production parameters.
When the quench fluid is contacted with the dope, the dope undergoes non-
equilibrium liquid-liquid phase separation to form a bicontinuous matrix of
large
interfacial area of two liquids in which the polymer rich phase is solidified
before
aggregated separation into distinct phases of small interfacial area can take
place.
Preferably, any air, gas or vapour (not being a gas or vapour that serves as
the
lumen fluid), is excluded during extrusion and the fibre is stressed axially
to stretch it by
a factor ranging from 1.5 to 5, thereby elongating the surface pores.
The hollow fibre membrane leaves the extrusion head completely formed and
there is no need for any further formation treatment except for removing the
solvent
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system from the membrane in a post-extrusion operation that is common to
membrane
manufacturing process. In a preferred method, an appropriate solvent that does
not
dissolve the polymer but is miscible with the dope solvents is used to remove
the
solvent system for the polymer from the finished membrane.
The lumen forming fluid may be selected from a wide variety of substances
such as are disclosed herein. The same substance may be used as the coating
and
quenching liquids. Water or virtually any other liquid may be used as the
quench liquid.
Water is used if a highly asymmetric structure is desired.
Asymmetric membranes can on rare occasions result from the TIPS process.
The rate and speed of de-mixing occurs faster at the outer surface of the
membrane and
slower further away from the interface. This results in a pore size gradient
with smaller
pores at the surface and larger pores further inwards. The pores at the
interface which
in a hollow fibre are the outer layer of the fibre and the wall of the lumen
may, in some
circumstances, be so small that a "skin" region occurs. This is about one
micron thick
and is the critical region for filtration. Thus, the outside of the fibre is
small pored
whereas the centre of the polymeric region has large pore size.
The initial poly (ethylene chlorotrifluoroethylene) membrane trials were
conducted by extrusion from small scale apparatus into a water quench, using
either
glycerol triacetate (GTA) or Citroflex 2 as the solvent. The structure of the
membranes as observed by SEM appeared to be excellent, although there was some
degree of skinning. The membrane prepared from Citroflex appeared the most
promising and had a relatively open skin with a number of larger holes.
A poly (ethylene chlorotrifluoroethylene) membrane was prepared by extrusion
in the manner described above for the TIPS process. The poly (ethylene
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chlorotrifluoroethylene) membranes were initially prepared without the use of
a coating
fluid, using GTA (table 1) or citroflex2 (table 2) as solvent.
TABLE 1. UNCOATED POLY (ETHYLENE
CHLOROTRIFLUOROETHYLENE) MEMBRANE - GTA SOLVENT
Parameter Value
Solvent 100% Glycerine Triacetate (GTA)
Lumen 100% Digol
poly (ethylene chlorotrifluoroethylene) 24%
Concentration
Barrel Temperature 230 C
Solvent injectors 230 C
Throughput 100cc/min
Screw speed 250rpm
Die Temperature 212 C
The dope was completely clear and homogeneous, indicating complete solubility
of the Halar in the GTA at 230 C. The dope solidified under ambient conditions
after
approx. 5 seconds. The fibre was extruded through a die at a temperature of
212 C into
a water quench. The air gap was approximately 15mm and the lumen forming
liquid
was diethylene glycol (digol).
Selecting a die temperature which is too low can lead to pulsing of the fibre
and
blockages in the die. Halar melts at 240 C and dissolves in GTA between 210 C
and
220 C with a cloud point around 215 C. The solvent was varied to Citroflex 2
as per
table 2
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TABLE 2 UNCOATED POLY (ETHYLENE
CHLOROTRIFLUOROETHYLENE) MEMBRANE - CITROFLEX2 SOLVENT
Parameter Value
Solvent 100% Citroflex 2
Lumen 100% Digol
poly (ethylene chlorotrifluoroethylene) 24%
Concentration
Barrel Temperature 230 C
Solvent injectors 230 C
Throughput 100cc/min
Screw speed 250rpm
Die Temperature 212 C
The dope was completely clear and homogeneous as with the GTA mixture,
indicating complete solubility of the polymer in Citroflex 2 at 230 C. The
dope had a
consistency slightly better than that of the GTA dope and also solidified
under ambient
conditions after approx. 5 seconds.
When Citroflex 2 was used as the solvent, it was necessary to add extra heat
to
the die to raise the temperature to sufficient levels to prevent blockages.
The fibre was
eventually extruded through a die at a temperature of approx. 212 C into a
water
quench. The air gap was approximately 15mm and the lumen liquid was diethylene
glycol (digol).
The SEMs showed the structure of the surface and of the cross-section of both
hollow fibre poly (ethylene chlorotrifluoroethylene) membranes prepared using
GTA
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and Citroflex 2 to have adequate pore formation and structure. The fibres were
also
surprisingly strong and ductile, with a large degree of flexibility.
The procedure was further modified by the use of a coating on the outside of
the
fibre. The use of coating compositions in the preparation of the Halar
membranes was
found to enhance the permeability (2200LMH) and improve the bubble point
(4901(Pa)
of the resultant membranes. The process parameters are shown below in table 3.
TABLE 3 COATED POLY (ETHYLENE
CHLOROTRIFLUOROETHYLENE) MEMBRANE VARIOUS SOLVENTS
Parameter Value
Solvent GTA
Coating GTA Citroflex 2 Digol
Lumen 100% Digol
Polymer Concentration 21%
Barrel Temperature 230 C
Solvent injectors 230 C
Throughput 100cc/min
Screw speed 250rpm
Die Temperature 200 C
As previously, the dope was clear and homogeneous, was of a good consistency
and solidified under ambient conditions after approx. 5 seconds. The fibre was
extruded
through a die at a temperature of approximately 200 C into a water quench. The
air gap
was approximately 15mm and the lumen liquid was diethylene glycol (digol).
It was necessary to ensure that the die temperature and a regular coating flow
were maintained. Irregular flow was minimised or eliminated by degassing the
coating
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and lumen vessels prior to use. Heated lines were installed for the coating
and lumen
fluids to help maintain die temperature. Extra insulation was also used, as
maintaining
an adequate temperature is required in order to produce a hollow poly
(ethylene
chlorotrifluoroethylene) fibre of consistent quality.
Two different trials were performed: GTA coating and Citroflex 2 coating. An
uncoated sample was produced for comparison.
TABLE 4 COATED POLY (ETHYLENE CHLOROTRIFLUORO
ETHYLENE) HOLLOW FIBRE MEMBRANE PERFORMANCE
Parameter No Coating GTA Coating Citroflex 2 Coating
% poly (ethylene 21 21 21
chlorotrifluoroethylene)
Coating Flow (cc/min) 0 10 10
Lumen Flow (cc/min) 5 5 5
Permeability (LMH 2294
@100kPa)
Bubble Point (kPa) 490
Break Extension (%) 92.9
Break Force (N) 1.35
Force/unit area (MPa) 4.6
Fibre OD/ID (pm) 856/469 766/461
As was apparent from the SEMs of the sample, the sample with no coating had
an impermeable skin, hence the absence of a result for permeability. The skin
also has
the effect of increasing break extension (BE) and break force (BF)
artificially therefore
these test were not performed either.
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The results from the GTA coated samples showed that permeability was high, as
was
break extension and force. In some cases, the photograph of the cross section
of the GTA
coated sample showed some small "holes", probably caused by bubbles in the
dope.
The high bubble point for the GTA sample indicates that many smaller pores
rather
than a smaller number of larger pores provide the high flow. The Citroflex 2
coated
membrane can be seen in the SEM's to have a good pore structure.
A summary of the Halar Trials membrane production results are given in Table
4a on
the following page 26A.
In order to produce membranes with a controlled density surface skin and
having a
more hydrophilic nature, silica was added to the dope with the intention of
subsequently
leaching the silica out of the formed membrane matrix by the use of a caustic
solution.
A hydrophilic silica, Aerosil R972 was tested as an additive to the poly
(ethylene
chlorotrifluoroethylene) membrane mixture. The dope was cast into a hollow
fibre
membrane, and the resultant hollow fibre membranes were quenched in water.
Once the membranes had been cast, a portion thereof was leached in a 5%
aqueous
caustic solution at room temperature for 14 hours.
After the membranes were cast, and prior to leaching, the membranes were
examined
using scanning electron microscopy. The structures were generally extremely
promising
with the surface of the sheets completely open and totally free of any skin.
The addition of the silica produced a hydrophilic membrane with a highly
porous
structure.
Subsequently placing the sample in caustic soda to leach the silica provided a
dramatic opening up in the membrane structure even further. The result of the
leaching was
a change in the cross-section from a conglomerate-like structure to the more
AKT/2801116
- 26A -
Table 4a
Haler Trials Summary
Sample %polymer Solvent Dope Lumen Lumen Stem Coating Coating Hauloff Quench
OD ID WT WT:OD Permeability %BE BF(N) BP(kPa) Stress
Flow Flow length Flow (m/min) Fluid (urn)
(urn) (urn) Ratio (LMH) (MAO
(cc/min) (cc/min) (cc./min)
1 21 GTA 22 Digol 5 none none nom 35 Water 856 469 193.5
0.23 - -
2 21 GTA 22 Digol 5 Short GTA 10 35 Water 766 461
152.5 020 2294 92.9 135 486 4.6 0
_
3 21 GTA 22 Digol ' 5 Short GTA 10 60 Water 775
481 147 0.19 2193 95.1 1.27 492 4.38 0
n.)
.o.
4 21 GTA 35 Digol 5 Long Cltroflex 10 35 Water 914 445
234.5 0.26 -4
.o.
(3)
2
n.)
ol
21 GTA 22 Digol 5 Long Citroflex 10 35 Water 802 486 158
0.20 n.)
o
1-,
1-,
2
1
o
w
O
-4
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traditional lace or sponge-like formation. The leaching with caustic soda
provided a
membrane of good open structure.
The optimal dope for forming a TIPS poly (ethylene chlorotrifluoroethylene)
polymer appears to be require the incorporation of 10-50wt% silica relative to
the
polymer.
A number of hollow fibre membranes were prepared from the above dope. The
wetting characteristics were as desired and the membrane structure showed an
extremely open surface. While 3-6% silica was used in the present invention,
it will be
appreciated that the quantity can vary significantly without departing from
the present
inventive concept.
Leaching the silica from the membranes had increased effect on the
permeability
and pore size of the hollow fibres without altering the desirable physical
properties of
the membrane.
A long leaching time is not necessarily required and can be incorporated in
the
production process as a post-treatment of the final modular product. The
leaching
process can be carried out at any time, however there is an advantage to
postponing the
leaching process as long as possible, since any damage to the surface of the
fibres
during handling can be overcome by leaching which physically increases the
porosity of
the membrane.
SEM analysis of the membranes showed a high degree of asymmetry.
Asymmetry is defined as a gradual increase in pore size throughout the
membrane
cross-section, such that the pores at one surface of the hollow fibre are
larger than the
other. In this case, the pore size increase was seen from the outer surface
where the
pores were smallest (and a quite dense surface layer was present) to the inner
surface
where the pores were significantly larger than those on the outer surface.
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As well as silica, the leaching process allows for the introduction of other
fimctionalities into the membrane, such as introducing hydrolysable esters to
produce
groups for anchoring functional species to membranes.
The leaching process has the capacity to maintain the hydrophilic character of
a
membrane after leaching. Again, without wishing to be bound by theory, the
silica
particles have a size in the order of nanometres so consequently the silica
disperses
homogeneously throughout the polymer solution. When the polymer is
precipitated in
the spinning process, there is a degree of encapsulation of the Si02 particles
within the
polymer matrix. Some of the particles (or the conglomerates formed by several
silica
particles) are wholly encapsulated by the precipitating polymer, some are
completely
free of any adhesion to the polymer (i.e. they lie in the pores of the polymer
matrix) and
some of the particles are partially encapsulated by the polymer so that a
proportion of
the particle is exposed to the 'pore' or to fluid transfer.
When contacted with caustic, it is believed that these particles will be
destroyed
from the accessible side, leaving that part of the particle in touch with the
polymer
matrix remaining. The remainder of the silica particle adheres to the polymer
matrix by
hydrophobic interaction and/or mechanical anchoring. The inside of the
particle wall is
hydrophilic because it consists of OH groups attached to silica. Because the
silica is
connected to hydrophobic groups on the other side, it cannot be further
dissolved.
Thus when the membranes are treated with caustic solution, the free
unencapsulated SiO2 reacts to form soluble sodium silicates, while the semi-
exposed
particles undergo a partial reaction to form a water-loving surface (bearing
in mind that
given the opportunity, such particles would have dissolved fully). It is
believed that the
pores in the polymer matrix formed during the phase inversion stage yet filled
with SiO2
particles are cleaned out during leaching, giving a very open, hydrophilic
membrane.
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Poly (ethylene chlorotrifluoroethylene) Membranes incorporating 3% Aerosil
R972 (fumed silica) into the membrane were prepared by the TIPS process. The
process parameters are given in Table 5. The poly (ethylene
chlorotrifluoroethylene)
fibre sample was then placed in an aqueous solution of 5wt% caustic to leach
the silica
from the membrane. The best result in terms of permeability was the Citroflex
coated
sample (11294LMH) but had a low bubble point (110kPa). The best result in
terms of
bubble point was the GTA coated sample (150kPa).
TABLE 5 COATED MEMBRANES WITH SILICA
Parameter Value
Solvent GTA
Coating None GTA Digol, Citroflex 2
Lumen 100% Digol
Polymer 21%
Concentration
Additives 3% (of dope) Aerosil R972 delivered as a slurry in
GTA
Barrel Temperature 230 C
Solvent injectors 230 C
Throughput 100cc/min
Screw speed 1250rpm
Die Temperature 200 C
The dope was similar to that produced in the earlier trials. The most obvious
difference was in opacity - with the silica included the dope was a cloudy
white colour.
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The fibre was extruded through a die at a temperature of approx. 200 C into a
water quench. The air gap was approximately 15mm and the lumen liquid was
diethylene glycol (digol).
Several different samples were taken. Some had no coating, others had GTA,
Digol and Citroflex 2 coatings applied at two different production rates (30
and
60m/min). The production parameters are shown in table 6
TABLE 6 COATED MEMBRANES WITH SILICA
Parameter No Coating GTA Digol Citroflex 2
% Polymer 21 21 21 21
% Aerosil R972 . 3 3 3 3
Coating Flow (cc/min) 0 10 10 10
Lumen Flow (cc/min) 5 5 5 5
Permeability (LMH 0 1354 1564 3296
@100kPa)
Bubble Point (kPa) 0 238 >50 155
Break Extension (%) - 118 52.3 71.1
Break Force (N) 1.81 1.30 0.86
Force/unit area (MPa) - 3.63 3.74 4.67
Fibre OD/ID (gm) 624/356 968/550 783/414 614/385
The SEMs show that even with silica in the membrane the use of no coating
agent resulted in the formation of a surface similar to a hollow fibre cast
without silica.
The appearance of the surfaces of the GTA and Citroflex hollow fibre membranes
are
similar, but the Citroflex coating gives a more open surface. This openness is
reflected
in the permeability and bubble point - the fibres coated with Citroflex have a
much
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lower bubble point and a much higher permeability than the GTA coated samples.
The
GTA and Citroflex coated membranes with Aerosil had a permeability close to
that of
the corresponding hollow fibre membrane samples prepared without added silica.
The Digol coated samples have a very rough and inconsistent surface, as shown
by the poor bubble point.
The samples described herein were are all prepared at a 30m/min production
rate. However, no significant difference was observed between 30, 60 and
100m/min
production rates in casting any of the samples.
The samples contain silica that can be leached from the fibres by the use of
caustic soda (sodium hydroxide). Thus the effect upon the flow rate and bubble
point
was determined by leaching an uncoated sample, a GTA coated sample and a
Citroflex
coated sample in 5wt% aqueous caustic solution at room temperature (23 C). The
Digol
sample was omitted from this process due to its poor properties. Table 7 below
gives
fibre results and the SEMs of the leached fibres follow.
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TABLE 7 RESULTS FOR LEACHED SILICA POLY (ETHYLENE
CHLOROTRIFLUOROETHYLENE) FIBRES
Parameter No Coating GTA Citroflex 2
% Polyrner 21 21 21
% Aerosil R972 3 3 3
Coating Flow (cc/min) 0 10 10
Lumen Flow (cc/min) 5 5 5
Permeability (LMH 5867 11294
@100kPa)
Bubble Point (kPa) 150 107
Break Extension (%) 115 81.0
Break Force (N) 1.67 0.98
Force/unit area (MPa) 3.36 5.43
Fibre OIDI1D (urn) 624/356 968/550 614/385
Post-leaching SEMs of the fibres show some very impressive structures. All of
the fibre cross sections are very open and in the case of the sample without
coating,
some asymmetry. The uncoated sample did not generate surface pores even after
5 days
of leaching in the case of 3% silica, although this may be overcome by
incorporating a
higher silica content in the dope mixture. The surfaces of any fibres are not
dramatically
altered after leaching, but there is a significant change in the porosity and
bubble point
of the fibres,
The Citroflex coated samples post-leaching increased in flow by nearly 350%
(3296 to 11294LMH) but the bubble point of the fibres while already low
dropped by
31% (154 down to 107kPa). This is consistent with the SEMs. The GTA samples
have
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been consistent with these results; the sample with Aerosil (pre-leaching) has
lost a
portion of its high bubble point (490 down to 2381(Pa) whereas permeability is
relatively
unchanged with the addition of Aerosil - as would have been expected for the
Citroflex
sample.
Post-leaching however gave a dramatic 320% increase in the flow (1354 up to
5687LMR) but a slightly larger drop in the bubble point of 37% (238 down to
150kPa).
The mean of the break extension (BE) and break force (BF) results for the GTA
and for the Citroflex coated samples were unchanged after 30-40hrs leaching in
5%
NaOH at room temperature. This shows the polymer and resulting membrane resist
caustic attack well.
The use of 3% silica was not sufficient to produce a hydrophilic membrane.
However it nevertheless opens up the membrane structure and improve flows.
With higher silica content, up to around 6%, the flow and bubble point do not
change dramatically from the results achieved with 3% Aerosil because the
presence of
the silica is most likely what induces the changes in the membrane structure,
not these
quantities. The surface of the fibre is also modified to get a better
retention.
The use of post treatment agents in modifying the properties of
ultrafiltration
membranes is known. One such post treatment, involving soaking the Halar
fibres in
50wt% aqueous glycerol solution for 24h was conducted. The results shown below
in
table 8 compare Halar fibres otherwise identical apart from the glycerol soak.
Soaking
was seen to dramatically increase the permeability of the membrane, from being
impermeable before treatment to having a permeability of 138Lm-2111 at 100Kpa.
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TABLE 8 POST SOAKING IN GLYCEROL
Parameter Halar Halar
No Post Treatment 50% Aqueous Glycerol 24h
Solvent 100% GTA 100% GTA
Coating 100% GTA 100% GTA
% Polymer 21 21
Coating Flow Rate (cc/min) 2.5 2.5
Lumen Flow Rate (cc/min) 5 5
Haul Off (m/min) 80 80
Permeability (Lm-2h1) No flow 138
@100kpa
Water Bubble Point (kPa) >660 >660
HFE Bubble Point (kPa) - 200-250
Break Extension (%) 131 131
Break Force (N) 1.14 1.14
Force/Unit Area (Mpa) 6.82 6.82
Fibre OD/ID 539/278 539/278
The ability of membrane synthesis methods to be scaled up to production levels
is important. The processes used to produce the large quantity of fibres must
not only
be operable on a small scale, they must also robust enough to be capable of
being scaled
up for use in a more typical production format, where solvent systems, die
design and
other production parameters need to be re optimised.
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Trials were initially conducted on a system used for the commercial
preparation
of PVDF membranes by a TIPS process. The main differences were the use of
PEG200
as the quench fluid, rather than water.
The production parameters are as shown in the following table 9.
TABLE 9 PRODUCTION PARAMETERS
Parameter Value
Solvent Citroflex 2
Coating Citroflex 2
Lumen 100%Digol
Polymer concentration 21%
Barrel Temperature 230 C
Solvent injectors 230 C
Throughput 100cc/min
Screw speed 250rpm
Die Temperature 230 C
As with the earlier trials, the extruder product was completely optically
clear and
homogeneous. The fibre was spun through a conventional TIPS die configurations
at a
temperature of 230 C, with a long (150mm) stem in which Citroflex 2 coated the
fibre.
Finally the fibre emerged into a glass tube with PEG200 as the quenching
media. There
was no air gap and the lumen liquid was diethylene glycol (digol).
The Trial produced fibres having the properties as shown in table 10.
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TABLE 10 ¨ CITROFLEX 2 COATED FIBRES
Parameter Citroflex 2 Coating
% Polymer 21
Coating Flow (cc/mm) 10
Lumen Flow (cc/min) 5
Permeability (LMH @1001(Pa) 2596
Bubble Point (kPa) 400
Break Extension (%) 145.8
Break Force (N) 1.3
Force/unit area (MPa) 8.38
Fibre OD/ID (urn) 626/439
The SEMs show a fibre with a morphology exhibiting a uniform cross section
with a
slight degree of asymmetry. Also apparent is a very coarse pore structure on
the surface,
with skinned areas in between. These skinned areas probably account for the
some of the
high break extension (BE).
This trial demonstrates that different quench liquids can be used to produce a
membrane with an acceptable structure. This is facilitated by the fact that
the Halar dope
is very close to the cloud point, enabling the use of most types of non-
solvent suitable to
the process as a quench fluid giving slightly different structures. However as
explained
below, given the good structure with water ¨ the cheapest non-solvent possible
¨ it does
not appear necessary to use another quench type.
A second trial was conducted with a similar dope using a triple head extruder
as
shown in figure lb. It is particularly preferred if the die is of a stem
configuration. In
figure lb, 13 is the coating fluid, 12 is the polymer solution (dope) and 11
is the lumen
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fluid. The stem can be of any length, but particularly is between 0.5 and
150mm so that
the coating covered the surface of the spun fibre evenly. The air gap, the
distance between
the die tip and the quench, can be any length but is most advantageously
between 0 and
lOmm. The production parameters are shown in the attached table.
TABLE 11 PRODUCTION PARAMETERS
Parameter Value
Solvent GTA, Citroflex 2
Coating GTA, Citroflex 2
Lumen 100% Digol
Polymer Concentration 21%
Barrel Temperature 230 C
Solvent injectors 230 C
Throughput 100cc/min
Screw speed 250rpm
Die Temperature 230 C
A plate was selected in preference to a long stem, the aim being to reduce the
contact
time between the coating fluid and the spun fibre. This was changed from 150mm
down
to ¨5mm of plate plus a very small air gap (-5mm) so that the coating contact
time is a
small as possible. Following this the fibre entered directly into a water
quench. Both the
temperature of the coating fluid and the total contact time have a significant
effect upon
the structure of the fibre surface.
The SEMs showed the fibres to exhibit a difference in the surface structure
compared
to the initial production trial. The temperature of the die and coating were
far more
accurately controlled in the present trials. The coating temperature in the
second trial was
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230 C 5 C, roughly 100 C above the coating temperature for the previous
trials. This
difference has a dramatic effect upon the membrane surface structure.
Several different samples were taken with GTA and Citroflex 2 coating at two
different production rates (30 and 60m/min). Samples with GTA as a solvent
were only
taken with a GTA coating and likewise for Citroflex 2. The results are shown
in table 12
and in the figures, which show representative examples of the membranes.
Figure 2 is a SEM which shows a Halar membrane prepared at a production rate
of
60m/min and coated with Citroflex at a rate of 7.5 cc/min.
Figure 3 is a SEM which shows a Halar membrane prepared at a production rate
of
80m/min and coated with GTA at a rate of 2.5 cc/min.
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TABLE 12 PRODUCTION PROPERTIES OF COATED MEMBRANES
Parameter Citroflex 2 GTA
% Polymer 21 21
Coating 5 7.5 10 5 7.5 1 2 5 2.5 2.5
Flow
(cc/min)
Lumen 5 5 5 5 5 5, 5 5 5 5
Flow
(cc/min)
Hauloff 60 60 60 80 80 60 60 60 80 100
(m/min)
Permeabilit 263 3515 3161 2366 309 38 19 64 - 57
y (LM-211-1 3 0
@100kPa)
Bubble 250
350 400 350 350 >660 >660 >660 >660 >660
Point (kPa)
Break 66 53 29 42 57 185 184 168 131 132
Extension
(%)
Break Force 0.9 0.84 0.71 0.74 0.6 1.36
1.26 1.45 1.14 1.26
(N) 6 9
Force/unit 6.7 3.63 4.35 2.49 2.0 4.87 7.50 5.20 6.82 7.56
area (MPa) 8 7
Fibre 652
621/ 570/ 660/ 561 710/ 760/ 697/ 539/ 535/
OD/ID(um) /37 336 380 376 /32 356 393 393 278 271
8 6
Unlike the results obtained in the initial trial, the surfaces here due to GTA
and
Citroflex are no longer similar and the Citroflex coating gives a less open
surface,
contrary to previous trials. This is most likely due to the increase in
coating temperature,
since at higher temperatures both the Citroflex,2 and GTA become more
aggressive as a
solvent. The Citroflex is most likely starting to re-dissolve some of the
surface of the fibre
before final precipitation is forced thus solidifying the structure.
The internal membrane structure also appears to be affected ¨ the pores
internally with
Citroflex 2 as a solvent appear far coarser than those in the structure with a
GTA solvent,
whose pores appear very small and tightly packed. This is reflected in the
permeability
and bubble point¨the fibres with Citroflex 2 as the solvent have a water
bubble point
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much lower (250-400kPa) but a much higher permeability (2500-3500LMH) than the
GTA coated samples. Given a regular surface on the Citroflex fibres the bubble
point
could be increased and the permeability enhanced.
The GTA samples are permeable however, at all coating flow rates. The GTA
samples
all had water bubble points far higher than the porometer could measure ¨ but
estimated
to be in the region 800-900kPa. These samples appear more clearly asymmetric
than the
samples with the Citro flex 2 as the solvent/coating.
The samples were tested for their capability for ultrafiltration. Initial
tests showed a
BFE bubble point of between 200 and 300kPa. This correlates to a membrane with
pores
approaching ¨ if not already within ¨ the UF range. Consequently one sample
was tested
for protein retention with Immuno Gamma Globulin (IGG, MW = 120kD). The sample
tested was the first of the GTA coated samples with 1 cc/mmn. of coating. The
sample
retained >95% of IGG, close to a known UF membrane possessing a retention of
98%.
These fibre samples were not treated with glycerol, as is standard practice
for UF-style
membranes. Glycerol prevents very small pores from collapsing upon drying the
membrane. Some similar samples to those UF tested were soaked in Glycerol
before
drying to prevent any possible pore collapse. This enhanced the permeability
of the
membrane up to 138 LMH from 0, and explains the poor penmeabilities in the UF
tests.
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TABLE 13 UF RESULTS
GTA solvent/Coating 1 cc/min Coating
Sample Time LMH
Ethanol 02:49:04 6.17
clean water 3:11:19.0 15.90
1 1:20:00.0 10.34
2 2:51:05.0 11.74
3 3:51:05.0 12.36
Figure 4 shows protein retention over time on a Halar membrane coated with GTA
at
lcc/min.
Both Citroflex 2 and GTA samples at 80m/min and the 100 m/min samples (GTA)
production rate show very little difference from the corresponding 60 m/min
samples in
flow surface structure, and no difference is apparent in either %BE, BF or
permeability.
Using GTA as a coating for the Halar fibres provides a remarkable amount of
control
over both the structure and porosity of the fibre surface. A lower coating
flow rate still
seems to keep the fibre permeable and enhances the asymmetry, whereas a higher
coating
flow rate gives a far more open surface. It is interesting is that the
permeability of the 1
cc/min samples is not vastly different from the 5 cc/min samples, yet the
fibre surface
appears far less porous. This suggests that the internal pore size is very
small. Thus if the
surface porosity is controlled accurately then either the polymer
concentration can be
decreased or Citroflex 2 used as a solvent to increase the permeability, all
while
maintaining excellent bubble point/retention characteristic of the fibre.
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FLAT SHEET PREPARATION
Approximately 160g of solvent (GTA or Citroflex 2) was placed into a glass
reaction vessel with a thermocouple to control the temperature. Stirring
continuously,
the solvent was heated to 230 C before approximately 40g of Halar 901LC was
added
to the vessel. The polymer dissolved rapidly and was allowed to mix for 10-15
minutes
before a sample of polymer solution was poured from the flask and onto a glass
plate
preheated to 120 C. The dope was then rapidly spread across the plate with a
glass bar
also preheated to 120 C. The bar had adhesive tape wound around the ends to
raise it a
uniform height above the plate when drawing the dope down, thus a sheet of
uniform
thickness was obtained. The cast membrane rapidly cooled and solidified to
form a flat
membrane sheet, which was washed in ethanol and dried in air.
VIRUS RETENTION RESULTS
A sample of Halar hollow fibre membranes were prepared in accordance with
the methods disclosed herein. The sample was prepared from a dope containing
Halar
901LC at a concentration of 21%, with a coating flow of 0.3m1/min. The
coating, the
solvent and the lumen were all GTA. The quench was in water at 15 C.
Dextran Retention:
Three to four fibres approximately 10cm long were made into a loop and the cut
ends sealed in epoxy glue. 148kd Molecular weight Dextran was filtered through
this
potted fibre. The feed & filtrate concentration was measured using HPLC and
the
percentage dextran retained by the fibre was calculated. Approximately 25% of
the
dextran was retained.
Virus Retention:
In a similar fashion, three to four fibres approximately 10cm long were made
into a loop and the cut ends sealed in epoxy glue. A solution of MS2 type
virus, at a
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feed concentration of approximately 30000 units per ml was filtered through
this potted
fibre. The log retention of virus was calculated and determined to be 4.30.
Typically, a
membrane having a viral log reduction of value of greater than 4 is considered
to be an
ultrafiltration membrane.
Permeability test:
The permeability of the fibres from the same batch as used for the dextran and
virus retention tests was also determined. Three to four looped and potted 10
cm fibres
were tested for permeability on a "porometer". The porometer allows water to
be
filtered at 100kPa pressure from the outside of the fibres to the inside and
out through
the fibre ends. The time required to pass 10m1 of water is recorded and used
to
calculate the permeability in litres/meter2.hour, which in the present case
was
determined to be 300 litreshneter2.hour.
The dextran, virus and permeability test were reproduced on a second batch of
Halar hollow fiber membranes prepared under identical conditions and identical
results
were obtained, suggesting that there were no reproducibility problems in the
use of
Halar to make ultrafiltration and microfiltration membranes.
Halar on its own forms a particularly good membrane with an excellent bubble
point and clean water permeability combined. The addition of coatings and
silica adds
another dimension to the membrane properties.
While the invention has been described with reference to particular
embodiments, it will be understood by those skilled in the art that the
inventive concept
disclosed herein is not limited only to those specific embodiments disclosed.
